This application claims the priority of Korean Patent Application No. 2002-16090, filed on Mar. 25, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an ohmic electrode for use in an optical device and a fabrication method thereof, and more particularly, to a p-type ohmic electrode in a gallium nitride based (GaN based) optical device and a fabrication method thereof.
2. Description of the Related Art
Optical devices, e.g., light emitting devices utilizing gallium nitride based compound semiconductors such as GaN, InGaN or AlGaN, effectively emit light with blue light wavelength region and have high light power compared to conventional optical devices. In particular, blue light has a shorter wavelength than red light, it can be easily converted into longer wavelength light. Thus, in recent years, a great deal of attention has been directed to GaN light emitting devices as white-light emitting devices.
In fabricating a GaN light emitting device, an n-type ohmic electrode and a p-type ohmic electrode are formed on an n-type gallium nitride (n-GaN) layer and a p-type gallium nitride (p-GaN) layer, respectively. The n-type and p-type ohmic electrodes must have low contact resistance and must be thermally stable. In particular, the p-type ohmic electrode must be a transparent electrode that can transmit emitted light. However, the p-type ohmic electrode formed on the p-type GaN layer has poor activation efficiency of Mg dopant doped into the p-type GaN layer, so that a hole concentration of 1018cmxe2x88x923 or higher cannot be obtained, making it difficult to attain contact resistance as low as approximately 10xe2x88x925 xcexa9cm2.
However, a p-type ohmic electrode in a general GaN light emitting device lowers contact resistance using a nickel (Ni)/gold (Au) layer formed by stacking an Au layer on an Ni layer. In this case, the contact resistance of the p-type ohmic electrode is as low as approximately 10xe2x88x924 xcexa9, the Ni layer is converted into a transparent NiO layer so as to easily transmit light, and the Au layer increases lateral transmittance of the electrode. The Ni/Au ohmic electrode having the entire thickness of approximately 10 nm has superior transmittance to light in the wavelength region of 300xcx9c500 nm, that is, approximately 80%.
However, even if the Ni/Au layer is used in the p-type ohmic electrode, the p-GaN layer itself has high resistance so that current from the p-type ohmic electrode cannot effectively spread into the p-GaN layer. In order to increase a current-spreading effect, the p-type ohmic electrode may be formed in a finger shape, which however results in a reduction of light emission.
Further, when the Ni/Au ohmic electrode is annealed for forming an ohmic contact, internal diffusion of Au may occur, suggesting poor thermal stability, and the lateral profile of the Au layer is not uniform.
To solve the above-described problems, the present invention provides a p-type ohmic electrode in a gallium nitride (GaN) based optical device having an increased emission efficiency with superior transmittance of light while having low contact resistance.
The present invention also provides a method of fabricating the p-type ohmic electrode in a gallium nitride (GaN) based optical device.
In an aspect of the present invention, there is provided a GaN based optical device including a substrate, a p-GaN layer formed on the substrate, and a p-type ohmic electrode formed on the p-GaN layer, wherein the p-type ohmic electrode is formed of a triple layer comprised of a nickel (Ni) layer, a gold (Au) layer and an indium tin oxide (ITO) layer sequentially formed. Preferably, the thicknesses of the Ni layer and the Au layer are smaller than the thickness of the ITO layer.
In another aspect of the present invention, there is provided a method of fabricating a GaN based optical device including forming a p-GaN layer on a substrate. Then, a metal layer pattern comprised of a nickel (Ni) layer, a gold (Au) layer and an indium tin oxide (ITO) layer sequentially formed is formed on the p-GaN layer. The metal layer pattern is annealed and then a p-type ohmic electrode is formed. The annealing of the metal layer pattern may be performed under an oxygen or nitrogen atmosphere. Also, the annealing of the metal layer pattern may be performed at 400xcx9c600xc2x0 C. In the metal layer pattern, the ITO layer may be formed using an RF magnetron sputtering apparatus while maintaining the temperature of the substrate at room temperature to 400xc2x0 C.
As described above, if the p-type ohmic electrode in the GaN based optical device with according to the present invention is formed of a triple layer comprised of Ni/Au/ITO, the Ni/Au layers in the triple layer reduces contact resistance and the ITO layer as a transparent, conductive oxide layer, increases transparency, and increases luminescence efficiency.